MOSFETs (Metal Oxide Semiconductor Field-Effect Transistors), in particular power MOSFETs, are widely used as electronic switches for switching electrical loads or as electronic switches in all types of switching converters. A power MOSFET includes a drain region, a drift region adjoining the drain region, and a source region, each having a first conductivity type, and a body region arranged between the drift region and source region of a second conductivity type. A gate electrode serves to control a conducting channel in the body region between the source region and the drift region. The source electrode is electrically connected to a source electrode which is also connected to the body region, and the drain region is electrically connected to the drain electrode. The MOSFET can be switched on and off by applying a suitable drive potential to the gate terminal.
In a specific type of MOSFET, which is also referred to as compensation or superjunction MOSFET, a compensation region is arranged in the drift region. This compensation region is of the same doping type as the body region and is electrically connected to the body region. The compensation region includes doping charges that are complementary to the doping charges in the drift region and that “compensate” the doping charges in the drift region when the MOSFET is in its off-state. By virtue of the compensation regions the drift region can be more highly doped than in conventional MOSFETs, resulting in a reduced on-resistance, at a given voltage blocking capability.
MOSFETs include a voltage dependent output capacitance (usually referred to as COSS) which usually includes a drain-source capacitance CDS between its drain and source terminals and a gate-drain capacitance CGD between its gate and drain terminals. When the MOSFET transitions from the on-state to the off-state, the output capacitance is charged, i.e. energy is stored in the output capacitance; the output capacitance is discharged, when the MOSFET transitions from the off-state to the on-state. The output energy EOSS, which is the energy stored in the output capacitance, is mainly dependent on the voltage across the drain-source path when the MOSFET is in its off-state and is dependent on the capacitance value of the output capacitance. A compensation MOSFET, due to the compensation regions connected to the body regions and the source electrode, has a high drain-source capacitance and, therefore, has a high output capacitance.
Losses occur when a MOSFET is operated. These losses mainly include capacitive losses and ohmic losses.
Capacitive losses are defined by the energy stored in the output capacitance of the MOSFET, wherein these losses increase with increasing output capacitance. In many applications, the capacitive losses dominate the switching losses under typical load conditions.
Ohmic losses occur when the MOSFET is in its on-state. Ohmic losses are due to the MOSFET's on-resistance. Additionally, switching losses occur when the MOSFET switches from the on-state to the off-state, and vice versa. These switching losses result from the fact that MOSFETs do not switch on or off abruptly, but they gradually change between the on-state, in which an ohmic resistance of the MOSFET assumes its minimum value, and the off-state, in which the MOSFET blocks and prevents a current flow. The minimum value of the ohmic resistance is the on-resistance.
The ohmic losses are proportional to the square of the load current, while the capacitive losses have a smaller dependency on the load current. Therefore, dependent on the specific load conditions, the ohmic losses or the capacitive losses may prevail. For example, when a load connected to the MOSFET draws a low load current, so that a low current flows through the MOSFET in its on-state, the capacitive losses may mainly determine the overall losses. Whereas, when the load draws a high load current, the ohmic losses and switching losses during transition phases may mainly determine the overall losses. The switching losses during transition phases and the capacitive losses are directly proportional to the switching frequency of the device.
In addition, the output charge QOSS, which is the charge stored in the output capacitance, is important for some applications. E.g., the turn off delay time of the MOSFET at low load currents is dominated by the output charge. This is the charge which has to be stored in the output capacitance before the transistor is completely turned off. This output charge is provided by the load current. Therefore, the turn off delay time increases inversely proportional with decreasing load current.
There is, therefore, a need to provide a MOSFET with a compensation region in which dependent on the load conditions the losses and turn off delay time can be minimized.